The present invention relates to a communications system having particular, but not exclusive, application to a short range (for example 0.5 to 3 km radius) hand portable mobile communications system in which a portable transceiver unit is capable of being called via a radio link from a base station connected to the public switched telephone network (PSTN) or is capable of making outgoing calls by way of a radio link to the base station.
Two known communications systems involving speech in which the first or final link in communications chain is by way of a duplex radio link are GSM (Groupe Special Mobile) digital cellular system and the DECT (Digital European Cordless Telecommunications) system. In the former case, that is GSM, a secondary or mobile station can make and receive telephone calls while roaming throughout the service area, if necessary at relatively high speeds of say 250 km/hr, through successive cells each having primary or base stations capable of call handover to the base station of the adjacent cell. The GSM system provides 8 duplex GSM channels per GSM carrier pair. Although GSM is a fully defined and functional digital cellular system it does have some disadvantages in that it is complex and the handset will be expensive. Additionally it is incapable of being used as a simple cordless telephone which would be cheaper to use.
The DECT system is a cordless telecommunications system in which the radio range is of the order of 100 m. DECT makes provision for contiguous radio coverage over local areas but not over larger metropolitan areas or nationwide. A portable unit therefore has to remain in range of the base station or a group of base stations through which a call is being made. Furthermore the portable unit is incapable of receiving incoming calls while outside the coverage region of the base station, although a portable unit can be alerted by a paging signal assuming that the portable unit incorporates a pager or the user is carrying a pager. As the peak effective radiated power of DECT portables is low, their range is limited and in consequence they cannot be extended to medium or large cells without extreme difficulty. The DECT system is a time division duplex (TDD) system having 12 duplex DECT channels per DECT carrier.